Renewing SSL certificates is one of those jobs which I end up researching for half an hour every time I try and do it. These instructions are geared towards SSL on Heroku.
On your local machine, create a new key pair:
openssl req -nodes -new -key server.key -out server.csr
heroku certs:update server.crt server.key -a app-name
Check everything is wired up properly with
curl -kvI https://www.example.com